Added service instance properties override feature

It allows to override specific service instances properties through the
definition of the new `serviceoverride` object. This is mainly useful for
dynamycally generated services such as those linked to host templates (those
defined in packs for instances), or linked to hostgroups. This allows to
override a specific instance parameters without having to define a complete
new service.
Imports `properties` attribute from Service class.
Implemented host property based service overrides.

Caution, it is taken the asumption that multi valued attributes are
implemented.
Forgot to update quick_tests.sh :)
Fix: merge issue.